The present invention generally relates to a ferromagnetic resonator device using ferrimagnetic resonance of a ferrimagnetic thin film and more particularly to a ferrimagnetic resonator having temperature compensation.
There has been proposed a ferromagnetic resonator for use in a microwave device as a filter or an oscillator. Such a ferromagnetic resonator is formed by forming a ferrimagnetic thin film, such as, YIG (Yttrium Iron Garnet) thin film through a liquid phase epitaxial growth on a nonmagnetic GGG (Gadolinium Gallium Garnet) substrate, and selectively etching the YIG thin film through a photolithographic process in a desired shape such as a disk shape or a rectangular shape. Such a microwave device has advantages that the microwave device can be formed in a MIC (microwave integrated circuit) having microstrip lines as transmission lines and that the microwave device can be connected easily with other MIC to form a hybrid circuit. Employment of a resonator element using an YIG thin film has advantages over a resonator element using an YIG sphere in that the YIG thin film can be formed through a mass-production process employing lithography techniques.
Such ferromagnetic resonator using a ferrimagnetic thin film has already proposed in U.S. Pat. Nos. 4,547,754, 4,636,756 and U.S. Ser. No. 844,984 filed Mar. 27, 1986 and now U.S. Pat. No. 4,679,015. Applications of such ferromagnetic resonator for a tuner and a oscillator are also proposed in U.S. Ser. No. 740,899 filed June 3, 1985, and now U.S. Pat. No. 4,704,739, and U.S. Pat. No. 4,626,800, all assigned to the assignee of the present application.
However, the ferromagnetic resonator employing a ferrimagnetic resonator element having an YIG thin film has a practical problem that the characteristics thereof is highly dependent on temperature.
The temperature characteristics of such a ferromagnetic resonator will be explained hereinafter.
The resonant frequency f of a ferrimagnetic resonator element employing, for example, an YIG thin film when a DC magnetic field is applied thereto in a direction perpendicular to the major surface of the YIG film is expressed by Kittel's equation: EQU f=.gamma.{Hg-(Nz-N.sub.T).multidot.4.pi.Ms(T)} (1)
on an assumption that the influence of the anisotropy field is negligibly small, where .UPSILON. is gyromagnetic ratio, which is 2.8 MHz/Oe for the YIG thin film, Hg is a DC bias magnetic field applied to the YIG thin film, Nz and N.sub.T are demagnetizing factors with respect to the direction of the DC magnetic field and a transverse direction, respectively, where (Nz-N.sub.T) is calculated on the basis of a magnetostatic mode theory, and 4.pi.Ms is the saturation magnetization of the YIG thin film, which is a function of temperature T. In a numerical example, Nz-N.sub.T =0.9774 for the perpendicular resonance of an YIG thin film having an aspect ratio (thickness/diameter) of 0.01. If the bias magnetic field Hg is constant regardless of temperature variation, the width of the range of variation of the resonant frequency f is as wide as 712 MHz in a temperature range of 0.degree. C. to +70.degree. C. because the saturation magnetization 4.pi.Ms of the YIG thin film is 1844G (Gauss) at 0 .degree. C. and 1584G at +70.degree. C.
We previously proposed YIG thin film microwave devices intended to solve the problems arising from the temperature characteristics in U.S. Ser. No. 708851 filed Mar. 6, 1985, and now U.S. Pat. No. 4,701,729, U.S. Ser. No. 883603 filed July 9, 1986, and U.S. Ser. No. 883605 filed July 9, 1986.
The temperature characteristics of the YIG thin film microwave devices we proposed are compensated by using a permanent magnet for applying a bias magnetic field to the YIG thin film resonator element according to the operating frequency of the ferrimagnetic resonator, or a bias magnetic circuit comprising a permanent magnet and a soft magnetic plate having a specific temperature coefficient. However, these inventions are applicable to YIG thin film microwave devices of a fixed frequency band type or of a narrow variable frequency band type, and are not capable of application to YIG thin film microwave devices of a widely variable frequency band type. That is, the temperature characteristics compensating method proposed in former Patent Applications had been developed on an assumption that the temperature of the YIG thin film and that of the permanent magnet or soft magnetic plate of the magnetic circuit are substantially the same.
However, an electromagnet having a coil to be energized to generate a magnetic field is employed instead of a permanent magnet, the heat generated by the energized coil causes comparatively large temperature difference between the YIG thin film and the magnetic circuit, and further between the components, for example, between the magnet and soft magnetic plate, of the magnetic circuits, and thereby the foregoing assumption becomes invalid
Accordingly, the foregoing temperature compensating method based on an assumption that the temperature of the ferrimagnetic resonator element and that of the magnetic circuit are on the same order is inappropriate to the ferromagnetic resonator of a widely variable frequency band type in which the magnitude of the current supplied to the electromagnet for applying a DC magnetic field to the ferrimagnetic resonator element is varied over a comparatively wide range.
Furthermore, in a strict sense or depending on the ambient conditions, the temperature of the ferromagnetic resonator element is different from that of the permanent magnet or the magnetic circuit also when the ferromagnetic resonator employs a permanent magnet for applying a DC bias magnetic field to the ferrimagnetic resonator element. Therefore, the temperature characteristics compensating method based on an assumption that there is no temperature difference between those components is not satisfactorily applicable even to the ferromagnetic resonator employing a permanent magnet.
A temperature compensating method for an oscillator employing a dielectric resonator is disclosed, for example, in 1984 IEEE MTT-S International Microwave Symposium Digest, pp. 277-279 (hereinafter referred to as "Reference 1"). This invention is based on an idea different from that of the Reference 1, which will become apparent from a description which will be given hereinafter.